Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount of a conjugated multiolefin. The preferred isoolefin is isobutylene.
Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a vehicle and a Friedel-Crafts catalyst as part of the polymerization initiator. The polymerization is generally carried out at temperatures of about −90° C. to −100° C. See U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288–295. The low polymerization temperatures are required in order to achieve molecular weights which are sufficiently high for rubber applications.
Other compounds that have been found to be active as catalysts for polymerizing isoolefins include organometallics compounds in combination with a cation-generating agent, such as C5Me5TiMe3/B(C6F5)3 (WO-00/04061-A1), Cp2AlMe/B(C6F5)3 (U.S. Pat. No. 5,703,182), and combinations of zirconocenes and related complexes with either B(C6F5)3 or CPh3[B(C6F5)4] (WO-95/29940-A1, DE-A1Song, X.; Thornton-Pett, M.; Bochmann, M. Organometallics 1998, 17, 1004, Carr, A. G.; Dawson, D. M.; Bochmann, M. Macromol. Rapid Commun. 1998, 19, 205.
Zinc compounds have not been used as catalysts for isoalkene polymerizations. Indeed, ZnCl2 in the presence or absence of alkyl halide activators (such as Me3CCl or MeCOCl) and used either in neat isobutene or in isobutene/methyl chloride mixtures, proves to be inactive, and no polymer is obtained.